The invention pertains to a device that automatically measures a characteristic impedance of a printed wiring circuit on a printed wiring board mainly using a coaxial probe.
A CPU clock frequency and an operation clock for CPU interaction with external devices recently has been remarkably increased so that a processing speed of a computer or the like may be increased. With the speed increase of the operation clock, the performance requirement for the printed wiring board where a data signal runs also become demanding. By way of example, there is a requirement that the characteristic impedance between printed wiring circuits formed by signal conductors on the printed wiring board need to be matched and that the characteristic impedance and the impedance of the circuit load need to be matched. Without the match, a signal is partially reflected back to a signal source such that the signal transmitted to the load becomes so weak that the data may not be transmitted completely. This problem becomes more serious with an increase of the frequency.
Thus, to maintain the quality of the printed wiring board, it will be essential to measure the characteristic impedance of the printed wiring circuit. Conventionally, the characteristic impedance of the printed wiring circuit is measured manually using a coaxial probe such as illustrated in FIG. 8. A coaxial probe has a center probe (201) and a cylindrical plunger with a protrusion (202) with a sharp tip as a part of the edge, which is provided to surround the center probe (201). It is briefly explained how to use a coaxial probe below. A through-hole that is connected to ground is usually provided around the end of a signal conductor on the printed wiring board that handles high frequency. The impedance of the coaxial probe is set to match the designed impedance of the measured printed circuit. First, the center probe (201) is brought in contact with the end of the signal conductor, and the protrusion (202) is brought in contact with the through-hole. Then, a predetermined high frequency signal is sent from the center probe (201), and the reflected wave signal from the circuit is received by the center probe (201). Now, the protrusion (202) is connected to ground. As the impedance of the coaxial probe and the impedance of the printed wiring circuit are designed to be equal, there should be no reflected wave signal if the circuit was manufactured according to the design. However, if there is deviation in the impedance of the printed wiring circuit from the designed one, the reflected wave is reflected at the part with the deviated impedance. Thus, if the period of time from when the high frequency signal is sent to when the reflected wave returns and the strength or amplitude of the reflected wave are measured, it is determined what level of the impedance deviation is present and at which part.
The operation clocks of equipment connected to computers are expected to increase in speed further in the future. Along with this trend, the need to measure the characteristic impedance to assure the quality of the printed wiring board will increase significantly. Then, the conventional method of manually measuring the impedance of the printed wiring circuit using a coaxial probe may not be good enough. It will be necessary to automate this process. Meanwhile, as a testing device for the printed wiring board, a device with a contact stylus automatically and two-dimensionally moved by a program to be in contact with the testing point of the printed wiring board to conduct a test for a line cut, etc. (hereafter called xe2x80x9can automatic printed wiring board testing devicexe2x80x9d) has been used conventionally. Thus, it is efficient and desirable to move a coaxial probe by means of this automatic printed wiring board testing device to test the testing point on the printed wiring board.
However, if a coaxial probe is fixed to the operating part of automatic printed wiring board testing device, the protrusion (202) may sometimes not be in contact with the through-hole when the center probe (201) is placed in contact with the edge of the signal conductor since the positional relationship between center probe (201) and the protrusion (202) will not be changed and since the position of a through-hole provided on the printed wiring board around the signal conductor is not fixed.
Besides, several different distances between the edge of the signal conductor and the through-hole may be used on the printed wiring board while the distance between the center probe (201) and the protrusion (202) of the coaxial probe is fixed. Thus, they may not match all the time. In some cases, when the center probe (201) is brought into contact with the edge of signal conductor, the protrusion (202) may not be in contact with the through-hole. Therefore, a good measurement may not be performed simply by applying the conventional coaxial probe to the automatic printed wiring board testing device.
The object of the present invention is to provide an impedance measuring device for the automatic measurement of the characteristic impedance of the printed wiring circuit on the printed wiring board principally by loading a coaxial probe in the automatic printed wiring board testing device.
The present invention seeks to solve the problem so as to provide an impedance measuring device for the printed wiring board that has a test probe unit such as a coaxial probe that includes a first and a second contact styli whose top tips point to the same direction wherein the second contact stylus is placed with a certain distance from the first contact stylus and a test probe driving means for moving the test probe unit to any place over the printed wiring board to be measured in order to make the first and the second contact styli contact certain measuring points on the printed wiring board, wherein the impedance measuring device further comprises a second contact stylus turning means for turning said second contact probe around said first contact stylus.
With this constitution, even when the direction is varied in which the through-hole is provided relative to the edge of signal conductor on the board to be tested, the second contact stylus turning means can turn the second contact stylus to match the direction of the second contact stylus in relation to the first contact stylus toward the direction of the through-hole in relation to the edge of the signal conductor. In this manner the first contact stylus may be brought into contact with the edge of the signal conductor, and at the same time the second contact stylus may be brought into contact with the through-hole so that the impedance of the signal conductor may be measured. Now, the impedance measuring device may be employed to measure the impedance with two styli contacted to two measuring points, although the impedance measuring of the signal conductor is shown as an example that is, the area of use of the device is not limited to the impedance measurement of signal conductors. It can be widely applied to any use wherein contact styli are placed in contact with two points to be measured. In this manner, with the impedance measuring device in accordance with the invention, the measurement can be made even when the direction of a point from the other is varied where the two points are provided for measurement. Therefore, the device can contribute to the realization of an automatic impedance measurement that uses a test probe unit such as a coaxial probe.
The second contact stylus turning means preferably turns the second contact stylus by turning the whole test probe unit around the first contact stylus as the central axis of turning. That is, if only the second contact stylus is designed to turn, a mechanism that enables the second contact stylus to turn independently form the first contact stylus movement needs to be provided in the vicinity of the top end of the test probe unit so that the mechanism must be small. On the other hand, if the whole test probe unit is designed to turn, the second contact stylus may turn around the first contact stylus as the center axis of turning without such a small mechanism as mentioned above.
The present invention further provides the impedance measuring device for the printed wiring board that has the test probe driving means which mounts the test probe unit that has the first contact stylus and the second contact stylus provided at a position apart from the first contact stylus by a certain distance with its tip facing the same direction as the first contact stylus tip, moves the test probe unit to any point over the board to be tested, and makes the first and second contact styli contact predetermined points of measurement on the board to be tested, wherein the impedance measuring device further comprises a replacement contact stylus holding means that holds a replacement second contact stylus for replacement and a contact stylus replacing means that replaces the second contact stylus of the test probe unit mounted on the test probe driving means with the replacement second contact stylus held by the replacement contact stylus holding means.
In an impedance measuring device that has this type of constitution, when the distance is varied between the edge of the signal conductor on the board to be tested and the through-hole, the contact stylus replacing means replaces the second contact stylus with a replacement (a replacement second contact stylus) having a corresponding distance between the first and the replacement second styli to the distance between the edge of the signal conductor and the through-hole as the contact stylus holding means holds the replacement second contact styli for replacement that have the distances from the first contact stylus corresponding to respective distances. In this manner, if the second contact stylus that has a distance corresponding to the distance between the edge of the signal conductor and the through-hole is used, the first contact stylus may be brought into contact with the edge of the signal conductor and at the same time the second contact stylus may be brought into contact with the through-hole so as to measure the impedance of the signal conductor. Now, although here the impedance measurement for signal conductors is taken as an example, it is just an example as mentioned before. In this manner the impedance measuring device in accordance with the present invention can make measurements even when there is more than one distance between two points to be measured. Therefore, it can also contribute to the realization of automatic impedance measurement using the test probe unit.
Furthermore, the replacement contact stylus holding means may have two or more contact stylus grasping means that grasp and release the second contact stylus that are driven by a driving means. The contact stylus replacement means may comprise the test probe driving means, the contact stylus grasping means, and a driving control means that controls the test probe driving means and the driving means for the contact stylus grasping means.
To explain briefly the operation of this example, it is premised that the test probe unit can be attached or detached by applying force to a specific part that contains the second contact stylus in a specific direction and that the replacement second contact stylus used also contains this specific part. First, there are at least one contact stylus grasping means (#1) that is open or empty or does not hold a replacement second contact stylus and one contact stylus grasping means (#2) that holds or grasps a replacement second contact stylus (#B). Then, with the contact stylus replacing means, the driving control means drives the test probe driving means to convey the second contact stylus (#A) of the test probe unit to a position such that it can be grasped by the empty contact stylus grasping means (#1) and that the second contact stylus (#A) is grasped by controlling the driving means of the contact stylus grasping means (#1). As the second contact stylus (#A) is grasped and fixed, the driving control means drives the test probe driving means to a specific direction to remove or detach the second contact stylus (#A) from the test probe unit. Then, the driving control means further drives the test probe driving means to install or attach the replacement second contact stylus (#B) on the test probe unit through a specific operation, where the replacement second contact stylus (#B) is grasped by the contact stylus grasping means (#2). When the replacement second stylus (#B) is installed, the driving control means controls the driving means of the contact stylus grasping means (#2) to open the grasping of the replacement second contact stylus (#B). Thus, the replacement of the second contact stylus (#B) is completed.
By adopting this constitution, as the test probe driving means can be commonly used as a part of the contact stylus replacing means, the structure of the whole device can be further simplified.
Furthermore, it is desirable to install in the impedance measuring device a turning position calibration means that calibrates the installation position in the turning direction of the second contact stylus around the first contact stylus of the test probe unit mounted on a test probe driving means as the central axis of turning. That is, if the installation position in the turning direction of the second contact stylus around the first contact stylus as the central axis of turning is displaced from the reference position, the second contact stylus is not correctly brought into contact with the point of measurement. Thus, it may prevent the device from applying to an automated operation. Also, turning or replacing of the second contact stylus may cause displacement of the installation position in the turning direction of the second contact stylus. Thus, the turning position calibration means can calibrate the installation position in the turning direction of the second contact stylus around the first contact stylus as the central axis of turning when it is needed to make more precise and accurate measurement.
It is premised that the second contact stylus comprises a ring that has a notched or missing part and is installed as the center axis of the ring is aligned to the center axis of the first contact stylus. The turning position calibration means may comprise a reference engaging part, whereby the test probe unit engages with the reference engaging part while the ring having a notched or missing part is inserted when the turning position of the second contact stylus around the first contact stylus is in a reference position.
With such constitution, the turning position of the second contact stylus may be finally adjusted by turning the second contact stylus as the notched part of the second contact stylus is directed to the reference engaging part such that the test probe unit is made to engage with the reference engaging part. In this manner, the calibration of the installation position in the turning direction of the second contact stylus is completed. That is, a turning position calibration means that has such a simple constitution that the position in the turning direction of the second contact stylus around the first contact stylus as the central axis of turning can be calibrated.
Also, the impedance measuring device may comprise a reference resistance that has a reference impedance that can be measured by the test probe unit. Since such reference resistance is installed, the measurement equipment can be tested whether it performs a correct measurement by measuring the reference resistance with the test probe unit when it is needed. Also, if the measurement equipment does not perform a correct measurement, the measurement setting can be adjusted based on the measurement result of this reference resistance in order to resume to perform a correct measurement.
Then, when the turning position calibration means contains a reference engaging part, it is more efficient to enable the test probe unit to measure the reference resistance during a period that the test probe unit engages with the reference engaging part. In this manner, when the test probe unit is engaged with the reference engaging part, both the calibration of the position in the turning direction of the second contact stylus around the first contact stylus as the central axis of turning and the calibration of the measurement equipment can be performed concurrently so that the operation time can be shortened.
A test probe unit may be used that is installed on the impedance measuring device and has an elastic member that pushes the first contact stylus toward the tip end direction. If such test probe unit is used, the impact between them can be mitigated by the elastic member when the first contact stylus hits the board to be tested. This is especially effective when the impedance measurement of the board to be tested is made automatically. Thus, damage and deterioration of the first contact stylus and the board to be tested can be prevented.
Also, the test probe unit that is used for the impedance measuring device equipped with the replacement contact stylus holding means and the contact stylus replacing means may be used whereby the second contact stylus is formed in a manner that can be attached to or detached from the first contact stylus in the longitudinal direction of the first contact stylus. When the test probe unit that has such constitution, only the part that contains the second contact stylus has to be replaced without replacing the first contact stylus. Thus, it is economical. Also, as the second contact stylus can be attached or detached by moving the first contact stylus in the longitudinal direction, the attachment and the detachment of the second contact stylus can be made by an axial directional movement of the first contact stylus. Since the axial directional movement is applied for the regular measurement by the test probe unit, no special mechanism for the operation is needed.
As mentioned earlier, the test probe unit used for the impedance measuring device with the turning position calibration means containing the reference engaging part may further have the ring with the notched part that engages with the reference engaging part. The test probe unit may be integrally installed with the second contact stylus and the ring having the central axis matched with that of the first contact stylus. Since this operation is the same as mentioned earlier, it is omitted here. By providing a simple structure for the test probe unit, the second contact stylus can be calibrated in the turning direction around the first contact stylus as the central axis of turning. Now, needless to say, the shape and the number of notches on the ring provided on the second contact stylus can be arbitrarily modified based on the needs.